1. Field of the Invention
The present invention is directed toward the field of manufacturing integrated circuits. The invention is more particularly directed toward treating deposited thin films that form integrated circuits to improve their electrical characteristics.
2. Description of the Related Art
Presently, aluminum is widely employed in integrated circuits as an interconnect, such as plugs and wires. However, higher device densities, faster operating frequencies, and larger die sizes have created a need for a metal with lower resistivity than aluminum to be used in interconnect structures. The lower resistivity of copper makes it an attractive candidate for replacing aluminum. One challenge in employing copper instead of aluminum is the fact that copper does not adhere well to materials, such as titanium nitride, that are presently being used as diffusion barriers beneath the copper. Diffusion barriers are deposited between layers of the devices to prevent cross-contamination. For example, material from a conducting layer can migrate into neighboring insulating layers thereby compromising the intended insulative characteristic. Such a condition can lead to shorting or performance degradation of the device. Poor adhesion of the copper to a diffusion barrier results in portions of the copper being undesirably peeled away during polishing. This condition can also render an integrated circuit defective.
One solution to improve the adhesion of copper to an underlying diffusion barrier is to divide the process for depositing copper into two steps. During a first step, physical vapor deposition (PVD) is performed to deposit a seed layer of copper. PVD is a well established technique for depositing material to create thin films. Once the seed layer of copper is deposited using PVD, a bulk layer of copper is deposited. The bulk layer is deposited by either standard chemical vapor deposition (CVD) or electrical plating. The bulk layer of copper adheres relatively well to the copper seed layer.
Unfortunately, the use of the PVD process results in poor step coverage, which is unacceptable for devices that have a high aspect ratio (the ratio of the depth of a feature to its cross sectional width on the substrate surface). Further, the PVD process cannot be accomplished in the same chamber as either chemical vapor deposition or electrical plating. The need to have both a PVD chamber and either a CVD or electrical plating chamber increases integrated circuit manufacturing costs and reduces throughput (the number of substrates processed per unit of time). Alternately, adhesion can be improved by treatment of the seed layer by bombardment with ions prior to bulk deposition of CVD copper. However such thin films have an undesirably high electrical resistivity. Therefore, it is therefore desirable to employ chemical vapor deposition, for depositing both the seed and bulk layers, because CVD provides for a more conformal layer of copper.
The chemical vapor deposition of copper presents a further challenge. The challenge arises from a byproduct that is produced during the deposition of the copper. In one instance, the chemical vapor deposition of copper is achieved by using a precursor known as Cupraselect, which has the formula Cu(hfac)L. The L represents a Lewis base compound, such as vinyltrimethylsilane (VTMS). The (hfac) represents hexafluoroacetylacetonato, and Cu represents copper. During the CVD of copper using the Cu(hfac)L precursor, the precursor is vaporized and flowed into a deposition chamber containing a substrate (i.e. a semiconductor wafer). In the chamber, the precursor is infused with thermal energy at the wafer's surface, and the following reaction results:
2Cu(hfac)L.fwdarw.Cu+Cu(hfac).sub.2 +2L (Eqn. 1)
The resulting copper (Cu) deposits on the upper surface of the wafer, along with the Cu(hfac).sub.2 byproduct. The gaseous Lewis base byproduct (2L) is purged from the chamber. The presence of the byproduct as well as other contaminants on the wafer's surface increases the resistivity of the copper to an underlying diffusion barrier, such as titanium or tantalum nitride. Post annealing after deposition of the final layer of copper reduces the resistivity further but also degrades the adhesion. Consequently, a post annealing step is not entirely advantageous to formation of copper films.
Adding moisture (H.sub.2 O) during the deposition of the seed layer by CVD increases the deposition rate which is highly desirable. Unfortunately, the addition of H.sub.2 O also produces contamination from excessive oxygen (O.sub.2) which reacts with the copper forming copper oxide (CuO). The CuO incorporated into the seed layer increases the resistivity of the resulting film to an undesirable level. Further, a bulk layer of copper will not adhere well to a CuO interface.
Accordingly, it is desirable to provide a method for the treatment of moisture laden copper CVD layers so that the adhesion between the copper and the underlying diffusion barrier is strong and the electrical resistivity is reduced. It is also desirable for such a deposition to be performed in a single chamber (in situ).